This invention relates to television picture tubes, and is concerned primarily with an electron gun having performance characteristics suitable for beam index tubes and other cathode ray tube displays requiring small beam spots.
Electron guns that can provide relatively small spot sizes; that is, beam spot diameters of the order of ten mils or less, find application in high-resolution monitors and in cathode ray tubes operating on the beam index principle. Beam index displays have long been touted as a viable alternative to color television picture tubes utilizing the shadow mask. Ideally, the beam-index color television tube provides full color reproduction of a transmitted television signal by means of a single beam, and without the need for a shadow mask. This is a decided advantage in terms of image brightness as the shadow mask typically intercepts about 80 percent of the beam energy.
A beam index system essentially comprises a screen consisting of a pattern of triplets of phosphor stripes: red, green, and blue; an electron gun capable of projecting a beam spot of a diameter approximating stripe width; means for sensing spot location in relation to the patterns of triplets; and means for modulating the beam with picture information.
More than two hundred patents have been issued in the field of beam index tubes including such significant disclosures as U.S. Pat. Nos. 2,415,059 to Zworykin; 2,545,325 to Weimer; 2,587,074 to Sziklai; 2,742,531 to Partin; and 2,892,123 to Sunstein. Interest in the beam index system has persisted because of the theoretical advantages over the widely used shadow mask system; the advantages include lower energy consumption, higher resolution, and in single-beam systems, no convergence problems.
However, the attainment of a viable beam index system able to compete with the shadow mask system has yet to be realized. While seemingly simple in concept, the beam index system in practice has proved to be fraught with problems. A major problem inherent in the beam index system is the need for additional, complex circuitry required to detect the position of the beam landing area relative to the individual triplets of red, green and blue stripes which provide for color reproduction. It is mandatory for color purity and verity that the proper luminance and chrominance information impressed on the beam be applied to the particular phosphor stripe on which the beam is currently landing. If the beam does not apply red information to a red stripe, for example, but red information to an adjacent series of green or blue stripes, the color will be scrambled in a way highly visible to the viewer.
Another major problem, and one to which the present invention is addressed, concerns beam spot size. For proper operation, the width of the scanning spot must be no greater than the width of each of the discrete stripes and its guardband on which the beam impinges, and this size relationship must be maintained at all points on the imaging screen. An additional exacerbation lies in the fact that the color stripe must be relatively narrow to provide acceptable image definition; e.g., preferably on the order of 10 mils, or less, in tubes of 13-inch diagonal measure.
A factor having a marked effect on spot size as the electron beam scans the picture imaging screen deposited on the face plate is the variance in the "throw" distance from the electron gun to a given point on the screen. The points of focus of the gun normally define an arc on the screen, while the area being scanned--the screen of the face plate--lies on an arc of much greater radius, hence is relatively planiform in comparison. As a result of scanning, the beam spot is undesirably gradationally enlarged from the minor axis to the minor sides of the face plate, culminating in an enlarged beam spot at the sides and the corners of the screen. As noted, it is mandatory that the width of the spot be substantially equal to the width of the stripes (and guardbands) if color purity is to be maintained. The enlargement of a beam spot at the sides of the screen, for example, may render it larger than the stripes/guardbands on which it is intended to impinge. The resulting overlap can result in hue and saturation errors. Also, a beam spot so enlarged can overlap a beam indexing stripe which can cause beam-indexing errors, as well.
Electron guns normally used in color television tubes are either the unipotential gun, which commonly has three electrodes in the main focusing lens, and the bipotential gun, which usually has a two-electrode main focus lens. The bipotential gun is in most common use. This type of gun normally has a relatively intermediate potential on the first, focusing electrode, and a relatively high potential on the second, accelerating electrode.
In UK Patent Application GB No. 2 020 092 A (RCA), there is disclosed a high potential, low magnification electron gun for use in color picture tubes. This bipotential-type gun comprises a cathode, an apertured plate control grid (Gl), an apertured plate screen grid (G2), and at least two tubular focusing electrodes (G3 and G4). It is stated that the quality of the gun's beam spot may be improved by (1) establishing an operating electric field between the G2 and G3 which is between about 100 and 400 volts/mil thereby reducing aberration effects in the beam-forming region of the gun; (2) making G2 thick so as to prevent the high G3 voltage from penetrating the region between G1 and G2, thereby allowing the G1-G2 field to provide a divergent effect on the electron beam prior to beam crossover and thus give a reduced crossover angle; (3) elongating G3 to provide an optimum filling of the main focus lens with a beam to maximize the object distance of the focusing system; and (4) structuring G2 and G3 to provide a flat electrostatic field therebetween to avoid prefocusing action in that region so as not to cause an effective reduction of the object distance of the focusing system. The focusing electrode (G3) potential is specified as being 8.5 kV and the accelerating anode (G4) potential is 30 kV.